refactor(preview): migrate to pkg/csvops library

Preview(ctx, PreviewOptions) (PreviewResult, error). Returns Headers and
Rows in memory — desktop UI pipes straight into a table view. Row-level
parse errors are returned via SkipErrors rather than printed, so callers
decide how to surface them.

Author: maherelgamil

cmd/preview.go

@@ -1,11 +1,11 @@
 package cmd
 
 import (
-	"encoding/csv"
+	"context"
 	"fmt"
-	"io"
 	"os"
 
+	"github.com/maherelgamil/csvops/pkg/csvops"
 	"github.com/olekukonko/tablewriter"
 	"github.com/spf13/cobra"
 )
@@ -24,37 +24,21 @@ var previewCmd = &cobra.Command{
 			return fmt.Errorf("please provide an input file using --input")
 		}
 
-		file, err := os.Open(previewInput)
+		res, err := csvops.Preview(context.Background(), csvops.PreviewOptions{
+			Input:     previewInput,
+			Rows:      previewRows,
+			NoHeader:  previewNoHeader,
+			Delimiter: ',',
+		})
 		if err != nil {
-			return fmt.Errorf("failed to open file: %w", err)
+			return err
 		}
-		defer file.Close()
 
-		reader := csv.NewReader(file)
-		reader.FieldsPerRecord = -1
-
-		var headers []string
-		if !previewNoHeader {
-			headers, err = reader.Read()
-			if err != nil {
-				return fmt.Errorf("failed to read header: %w", err)
-			}
-		}
-
-		rows := [][]string{}
-		for len(rows) < previewRows {
-			record, err := reader.Read()
-			if err == io.EOF {
-				break
-			}
-			if err != nil {
-				fmt.Printf("⚠️  Skipping row due to error: %v\n", err)
-				continue
-			}
-			rows = append(rows, record)
+		for _, e := range res.SkipErrors {
+			fmt.Printf("⚠️  Skipping row due to error: %v\n", e)
 		}
 
-		if len(rows) == 0 {
+		if len(res.Rows) == 0 {
 			fmt.Println("⚠️  No data rows found")
 			return nil
 		}
@@ -65,15 +49,15 @@ var previewCmd = &cobra.Command{
 		table.SetAlignment(tablewriter.ALIGN_LEFT)
 		table.SetRowLine(true)
 
-		if !previewNoHeader {
-			table.SetHeader(headers)
+		if len(res.Headers) > 0 {
+			table.SetHeader(res.Headers)
 		}
-		for _, row := range rows {
+		for _, row := range res.Rows {
 			table.Append(row)
 		}
-
 		table.Render()
-		fmt.Printf("\n📄 Showing %d row(s) from '%s'\n", len(rows), previewInput)
+
+		fmt.Printf("\n📄 Showing %d row(s) from '%s'\n", len(res.Rows), previewInput)
 		return nil
 	},
 }

pkg/csvops/preview.go

@@ -0,0 +1,79 @@
+package csvops
+
+import (
+	"context"
+	"encoding/csv"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+)
+
+// PreviewOptions configures a Preview operation.
+type PreviewOptions struct {
+	Input     string
+	Rows      int  // max rows to return
+	NoHeader  bool // if true, the first row is treated as data
+	Delimiter rune
+}
+
+// PreviewResult is returned from Preview.
+type PreviewResult struct {
+	Headers   []string   // empty when NoHeader is true
+	Rows      [][]string // up to opts.Rows rows
+	SkipErrors []error   // per-row parse errors that were skipped
+}
+
+// Preview reads the first Rows data rows of the CSV and returns them in memory.
+// Unlike the other ops it does not stream — callers expect a bounded slice.
+func Preview(ctx context.Context, opts PreviewOptions) (PreviewResult, error) {
+	var res PreviewResult
+
+	if opts.Input == "" {
+		return res, fmt.Errorf("input is required")
+	}
+	if opts.Rows <= 0 {
+		opts.Rows = 5
+	}
+	if opts.Delimiter == 0 {
+		opts.Delimiter = ','
+	}
+
+	f, err := os.Open(opts.Input)
+	if err != nil {
+		return res, fmt.Errorf("open input: %w", err)
+	}
+	defer f.Close()
+
+	reader := csv.NewReader(f)
+	reader.Comma = opts.Delimiter
+	reader.FieldsPerRecord = -1
+
+	if !opts.NoHeader {
+		h, err := reader.Read()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				return res, nil
+			}
+			return res, fmt.Errorf("read header: %w", err)
+		}
+		res.Headers = h
+	}
+
+	res.Rows = make([][]string, 0, opts.Rows)
+	for len(res.Rows) < opts.Rows {
+		if err := ctx.Err(); err != nil {
+			return res, err
+		}
+		rec, err := reader.Read()
+		if errors.Is(err, io.EOF) {
+			break
+		}
+		if err != nil {
+			res.SkipErrors = append(res.SkipErrors, err)
+			continue
+		}
+		res.Rows = append(res.Rows, rec)
+	}
+	return res, nil
+}

pkg/csvops/preview_test.go

@@ -0,0 +1,71 @@
+package csvops
+
+import (
+	"context"
+	"path/filepath"
+	"reflect"
+	"testing"
+)
+
+func TestPreview_WithHeader(t *testing.T) {
+	dir := t.TempDir()
+	in := filepath.Join(dir, "in.csv")
+	writeCSV(t, in, "id,name\n1,a\n2,b\n3,c\n4,d\n")
+
+	res, err := Preview(context.Background(), PreviewOptions{Input: in, Rows: 2})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(res.Headers, []string{"id", "name"}) {
+		t.Errorf("headers = %v", res.Headers)
+	}
+	if len(res.Rows) != 2 {
+		t.Errorf("rows = %d, want 2", len(res.Rows))
+	}
+	if !reflect.DeepEqual(res.Rows[0], []string{"1", "a"}) {
+		t.Errorf("row[0] = %v", res.Rows[0])
+	}
+}
+
+func TestPreview_NoHeader(t *testing.T) {
+	dir := t.TempDir()
+	in := filepath.Join(dir, "in.csv")
+	writeCSV(t, in, "1,a\n2,b\n")
+
+	res, err := Preview(context.Background(), PreviewOptions{Input: in, Rows: 5, NoHeader: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(res.Headers) != 0 {
+		t.Errorf("expected no headers, got %v", res.Headers)
+	}
+	if len(res.Rows) != 2 {
+		t.Errorf("rows = %d, want 2", len(res.Rows))
+	}
+}
+
+func TestPreview_AsksForMoreThanAvailable(t *testing.T) {
+	dir := t.TempDir()
+	in := filepath.Join(dir, "in.csv")
+	writeCSV(t, in, "h\n1\n")
+	res, err := Preview(context.Background(), PreviewOptions{Input: in, Rows: 100})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(res.Rows) != 1 {
+		t.Errorf("rows = %d, want 1", len(res.Rows))
+	}
+}
+
+func TestPreview_EmptyFile(t *testing.T) {
+	dir := t.TempDir()
+	in := filepath.Join(dir, "in.csv")
+	writeCSV(t, in, "")
+	res, err := Preview(context.Background(), PreviewOptions{Input: in, Rows: 5})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(res.Headers) != 0 || len(res.Rows) != 0 {
+		t.Errorf("empty file should yield empty result, got %+v", res)
+	}
+}